# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param head ListNode类 
# @return ListNode类
#
class Solution:
    def deleteDuplicates(self , head: ListNode) -> ListNode:
        # write code here
        if not head:
            return head
        dummy = ListNode(-1)
        dummy.next = head
        pre, cur = head, head.next
        # 双指针，两层循环
        while pre:
            cur = pre.next
            while cur:
                if pre.val == cur.val: 
                    pre.next = cur.next 
                cur = cur.next 
            pre = pre.next 
        return dummy.next